It's dark almost on time (5)
I believe the answer is:
night
'it's dark' is the definition.
(night is a kind of dark)
'almost on time' is the wordplay.
'almost' becomes 'nigh' ('nigh' can be a synonym of 'almost').
'on' means one lot of letters go next to another.
'time' becomes 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "What proverbially follows day" , "Evening" , "See 9" , "follow-up to 16 [TWILIGHT]" , "See 1 Across" .)